The present application relates to a cathode active material as a lithium-containing compound, to a cathode and a secondary battery that use the cathode active material, and to a battery pack, an electric vehicle, an electric power storage system, an electric power tool, and an electronic apparatus that use the secondary battery.
In recent years, various electronic apparatuses such as a mobile phone and a personal digital assistant (PDA) have been widely used, and it has been demanded to further reduce the size and the weight of the electronic apparatuses and to achieve their long life. Accordingly, as an electric power source for the electronic apparatuses, a battery, in particular, a small and light-weight secondary battery capable of providing high energy density has been developed. In these days, it has been considered to apply such a secondary battery to various other applications in addition to the foregoing electronic apparatuses. Representative examples of such other applications include a battery pack attachably and detachably mounted on the electronic apparatuses or the like, an electric vehicle such as an electric automobile, an electric power storage system such as a home electric power server, and an electric power tool such as an electric drill.
Secondary batteries utilizing various charge and discharge principles to obtain a battery capacity have been proposed. In particular, a secondary battery utilizing insertion and extraction of an electrode reactant or a secondary battery utilizing precipitation and dissolution of an electrode reactant has attracted attention, since such a secondary battery provides higher energy density than lead batteries, nickel-cadmium batteries, and the like.
The secondary battery includes a cathode, an anode, and an electrolytic solution. The cathode contains a cathode active material contributing to a charge and discharge reaction. As the cathode active material, generally, a lithium-containing compound such as LiCoO2, LiNiO2, and Li(Ni0.5Co0.2Mn0.3)O2 is widely used.
In the development field of the secondary battery, increasing the charging voltage to a value larger than 4.2 V has been considered to fulfill a need for achieving a high-capacity of the secondary battery. However, in the case where the charging voltage is increased, a lithium-containing compound is easily degraded. Thereby, cycle characteristics as one of the important characteristics of the secondary battery are lowered. Therefore, it is substantially difficult to increase the charging voltage at present. Accordingly, as a cathode active material for a high charging voltage, a lithium-rich lithium-containing compound represented by a general formula Li1+a[MnbMn1−b]1−aO2−c has been proposed (for example, see Electrochemical and Solid-State Letters, 9(5), A221-A224 (2006)). In the formula, M represents a transition metal element (excluding Mn), and 0<a<0.25, 0.3≦b<0.7, and −0.1≦c≦0.2 are satisfied. In the case where the lithium-rich lithium-containing compound is used, a discharge capacity more than about 250 mAh/g is obtained.
In the case where the lithium-rich lithium-containing compound is used, the cycle characteristics and/or the like is improved. On the other hand, lithium ions are less likely to be inserted and extracted mainly due to increased resistance, and therefore, load characteristics are lowered. Since such a disadvantage of the lowered load characteristics may be an obstacle to practical use thereof, various studies have been made to improve the load characteristics.
Specifically, in a high-lithium-containing transition metal composite oxide particle represented by a general formula Li1+x−sMn1−x−yMyO2−t, the crystal structure of the particle is changed from a layered structure to a spinel structure as the position thereof gets from the center side to the surface side (for example, see Japanese Unexamined Patent Application Publication No. 2011-096626). In the formula, M represents a transition metal element other than Mn, and 0<x<0.33, 0<y<0.66, 0<s<0.3, and 0<t<0.15 are satisfied.
In a lithium-manganese composite oxide having a spinel-type crystal structure represented by a general formula Li1+XMn2−Y−ZMZO4+δ, the average particle diameter is from 5 μm to 20 μm both inclusive, the BET specific surface area is equal to or less than 1 m2/g, and the average crystallite diameter is equal to or more than 100 nm (for example, see Japanese Unexamined Patent Application Publication No. 2002-226214). In the formula, M represents Ni or the like, and 0≦X≦1/3, 0≦Y≦1/3, 0<Z≦0.25, and −0.14≦δ≦0.5 are satisfied.
In a lithium-manganese composite oxide having a spinel-type structure represented by a general formula LixMn2Oy, the BET specific surface area is equal to or more than 2 m2/g, and the crystallite diameter is equal to or more than 30 nm (for example, see Japanese Unexamined Patent Application Publication No. H08-002921). In the formula, 1<x<1.6, 4<y<4.8, (8/3+4/3×x)<y<(4+1/2×x) are satisfied.
In a lithium-transition-metal composite oxide having a spinel-type (Fd3-m) represented by a general formula Li1+xM2−xO4, the specific surface area is from 0.35 m2/g to 0.8 m2/g both inclusive, and the crystallite size is from 170 nm to 490 nm both inclusive (for example, see WO2009/054436). In the formula, M represents Mn or the like, and 0.01≦X≦0.08 is satisfied.
Regarding a manganese composite oxide, a nickel composite oxide, and a cobalt composite oxide, the specific surface areas are equal to or more than 5 m2/g, and the crystallite diameters are equal to or less than 70 nm (for example, see Japanese Unexamined Patent Application Publication No. 2006-202724).
In a lithium-transition-metal composite oxide having a spinel-type (Fd3-m) represented by a general formula Li(LixMgyAlzMn2−x−y−z)O4, the specific surface area is from 0.35 m2/g to 0.8 m2/g both inclusive, and the crystallite size is from 170 nm to 490 nm both inclusive (for example, see Japanese Unexamined Patent Application Publication No. 2010-097947). In the formula, 0.01≦x≦0.08, 0.02≦y≦0.07, and 0.06≦z≦0.14 are satisfied.